Nowadays, more than one Global Navigation Satellite System (GNSS) is available, which includes GPS (Global Positioning System), Galileo and GLONASS (GLObal NAvigation Satellite System). GPS uses CDMA (Code Division Multiple Access). That is, in GPS, satellites are distinguished from each other by modulating signals of the respective satellites with different PRN codes. GLONASS uses FDMA (Frequency Division Multiple Access). That is, in GLONASS, satellites are distinguished from each other by using different carrier frequencies. Table 1 shows GLONASS carrier frequencies in L1 and L2 sub-bands.
TABLE 1GLONASS carrier frequencies in L1 and L2 sub-bandsNominal valueNo. ofof frequencyNo. ofNominal value of frequencychannelin L1 sub-band, MHzchannelin L2 sub-band, MHz131609.3125131251.6875121608.75121251.25111608.1875111250.8125101607.625101250.375091607.0625091249.9375081606.5081249.5071605.9375071249.0625061605.375061248.625051604.8125051248.1875041604.25041247.75031603.6875031247.3125021603.125021246.875011602.5625011246.4375001602.0001246.0−011601.4375−011245.5625−021600.8750−021245.1250−031600.3125−031244.6875−041599.7500−041244.2500−051599.1875−051243.8125−061598.6250−061243.3750−071598.0625−071242.9375
FIG. 1 is a block diagram schematically and generally shows a basic structure of a modern GNSS receiver 100 in accordance with related art. The receiver 100 includes an antenna 101, an RF front end 112, an IF down-converter 123, a local oscillator 128, a correlator engine 130, a correlator memory 135, a local code generator 147 and a processor 150. The receiver 100 receives a satellite signal in RF band via the antenna 101. The received RF signal is down converted into an IF signal in the RF front end 112, which also amplifies the signal. The IF signal is passed to the IF down-converter 123. The IF down-converter 123 down converts the IF signal into a baseband signals by using an IF carrier provided by the local oscillator 128. The baseband signal is passed to the correlator engine 130 to be correlated with a code provided by the local code generator 147. The correlation result is stored in a correlator memory 135 for accumulation. The processor 150 processes the correlation result and/or the accumulation of the correlation result to generate PVT (position-velocity-time) information. In this structure, the IF carrier frequency can only be a fixed value. However, in practice, spread spectrum signals from different satellites (e.g. GLONASS mentioned above) or different GNSS systems may use different carriers. That is, the carrier frequencies of the incoming spread spectrum signals are different. As a result, multiple IF carrier removal modules are used in a current receiver such as a GLONASS receiver to improve satellite searching and tracking efficiency. Each of the IF carrier removal modules is for a specific carrier frequency.